The symmetric structure shown in the figure is the right half of a T-Pad
attenuator manufactured by Barth Electronics.
The input signal passes through a long horizontal resistor which forms
a voltage divider with the short vertical resistor. The horizontal resistor
on the other side is connected to the output. The two horizontal resistors
have a value such that the resistance looking into each port equals 50
Ohms to match the cables attached to the input and output ports. The attenuation
is set by the value of the short vertical resistor.

Along each horizontal resistor, the resistance to ground decreases along
the resistor as one moves from the input/output port to the junction of
the resistors. At either port, the impedance to ground of this particular
attenuator is 50 Ohms and at the junction of the horizontal and vertical
resistors it is about 15 Ohms. The walls of the attenuator are tapered
so as to cause the characteristic impedance of the transmission line formed
by the horizontal resistor and the cavity walls to be equal to the resistance
to ground along each horizontal resistor. Such a design technique results
in a device with flat frequency response from DC to tens of GHz.